Semiconductor device

ABSTRACT

A thin film transistor of the present invention has an active layer including at least source, drain and channel regions formed on an insulating surface. A high resistivity region is formed between the channel region and each of the source and drain regions. A film capable of trapping positive charges therein is provided on at least the high resistivity region so that N-type conductivity is induced in the high resistivity region. Accordingly, the reliability of N-channel type TFT against hot electrons can be improved.

BACKGROUND OF THE INVENTION

The preset invention relates to a semiconductor device and amanufacturing method for the same. In particular, the present inventionis directed to an insulated gate field effect transistor (TFT) formed onan insulating surface, for example, a surface of an insulating substratesuch as glass, or an insulating film such as silicon oxide formed on asilicon wafer. Also, the present invention is advantageous for theformation of an insulated gate field effect transistor, especially of anN-channel type, which is driven at a relatively high voltage. It is alsoto be understood that the present invention is further advantageous forthe formation of a TFT on a glass substrate of which glass transitiontemperature (i.e. distortion point) is 750° C. or lower.

Moreover, the present invention is related to an active matrix of aliquid crystal device, a driving circuit of an image sensor or a threedimensional integrated circuit (hybrid IC) using the foregoingsemiconductor devices.

In the prior art, TFTs have been known for driving an active-matrix typeliquid crystal device or an image sensor or the like. Specifically, inplace of an amorphous TFT using an amorphous silicon as an active layer,crystalline TFTs having a higher mobility are now being developed inorder to increase driving speed. Moreover, TFTs having a highresistivity region (high resistivity drain) in an active region thereofhave been proposed in order to further improve the devicecharacteristics and to increase the capability of driving with a highervoltage. The “high resistivity region” or “high resistivity drain” inthe present invention includes an impurity region (drain) having ahigher resistivity, a lightly doped drain (LDD), and also an offsetregion where a gate electrode does not overlap an impurity region.

However, negative charges caused by hot carriers in an N-channel typeTFT tend to be trapped in a portion of a gate insulating film close to adrain region so that the conductivity type of the high resistivityregion shifts to p-type. As a result, the source/drain current isobstructed.

Also, it is necessary to use a photolithography technique to form a highresistivity region. This means that production yield and a uniformity ofcharacteristics in the obtained TFTs can not be improved.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the quality of TFTsand the manufacturing yield by solving the foregoing problems.Specifically, it is an object of the present invention to preventdegradation caused by hot carriers, and to produce a high resistivityregion in a self-aligning manner without using a photolithographyprocess.

It is a further object of the present invention to manufacture a liquidcrystal device using the TFTs of the present invention.

It is still another object of the present invention to produce TFTswhich have a high resistivity against water which tends to be containedin an interlayer insulator, especially, formed from TEOS gas.

It is still another object of the invention to utilize electricalcharges occurring in an interlayer insulating film to stabilize theproperty of TFTs.

In accordance with the present invention, a TFT comprises an activesemiconductor layer including at least source, drain and channelregions, and further a high resistivity region between the source andchannel regions and/or the drain and channel regions, wherein a filmwhich is capable of trapping positive charges is formed adjacent to thehigh resistivity region. FIG. 1 shows a typical example of thisstructure.

In FIG. 1, an N− region 111 is interposed between a source region 110having an N-type and a channel region 3. A gate insulating film 104exists on the N− region 111. Further, a silicon nitride film 114 whichis capable of trapping positive ions therein is formed on the sourceregion and the gate insulating film 104. It is to be understood thateven if hot electrons are injected into the gate insulating film fromthe active layer close to the source region, these can be neutralized bythe positive charges existing in the silicon nitride film 114.Accordingly, the high resistivity region can function correctly. Also,the TFT shown in FIG. 1 includes an offset region between the channelregion 3 and the high resistivity region 111. The offset region is anextension of the channel region and has a same conductivity type as thechannel region (intrinsic).

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and features of the invention will be described inpreferred embodiments of the invention with reference to the attacheddrawings in which:

FIG. 1 shows a partial cross sectional view of a TFT in accordance withthe present invention;

FIGS. 2A to 2D show cross sectional views of TFTs in accordance with apreferred embodiment of the present invention;

FIGS. 3A to 3H show a manufacturing process of a TFT in accordance witha first example of the invention;

FIGS. 4A to 4C show a manufacturing process of a TFT in accordance witha second example of the invention;

FIGS. 5A to 5C show examples of monolithic circuits employing TFTs inaccordance with the present invention;

FIGS. 6A to 6F show a manufacturing process of TFT in accordance withthe third example of the invention;

FIGS. 7A to 7F show a manufacturing process of TFT in accordance withthe fourth example of the invention;

FIGS. 8A to 8F show a manufacturing process of TFT in accordance withthe fifth example of the invention;

FIGS. 9A to 9G show a manufacturing process of TFT in accordance withthe sixth example of the invention;

FIG. 10 is a schematic diagram showing a liquid crystal device inaccordance with the present invention; and

FIGS. 11A to 11D show a manufacturing process of TFT in accordance withthe seventh example of the invention.

PREFERRED EMBODIMENT OR THE INVENTION

In a preferred embodiment of the invention, a film having a capabilityof trapping positive charges, for example, silicon nitride is formed ona high resistivity region in direct contact therewith or with a gateinsulating silicon oxide film interposed therebetween. The thickness ofthe positive charge trapping layer is, for example, 200-2000 Å. Thepositive charges trapped in the film make the conductivity type of thehigh resistivity region adjacent thereto slightly N-type, or neutralizenegative charges injected into the gate insulating film, thereby,avoiding a degradation caused by hot carriers. For example, whenapplying a +15 V to a drain and −20V to a gate, negative charges causedby impact ionization are not trapped by a silicon oxide film when thesilicon oxide film does not exist on the high resistivity region. Alsowhen the silicon oxide film exists on the high resistivity region andthe negative charges are trapped therein, the charges will beneutralized by the positive charges. Therefore, it is possible toprevent the high resistivity region from becoming P-type.

FIGS. 2A-2D show various examples of locational relations between thepositive charge trapping layer and the gate insulating film. In FIG. 2A,the TFT has a channel region 3, source and drain regions 1 and 5, a gateinsulating film 6, a gate electrode 7, an anodic oxide film 12surrounding the gate electrode and further a pair of high resistivityregions 2 and 4. Also, an interlayer insulator 8 is formed on the uppersurface of the TFT and source and drain electrodes 9 and 10 are providedtherethrough. Further, a charge trapping film 11 is provided as shown inthe figure.

Specifically, in FIG. 2A, the gate insulating film 6 covers the entireactive layer including the source and drain regions 1 and 5. The chargetrapping layer 11 is superimposed on the gate insulating film.

In FIG. 2B, the gate insulating film 6 extends beyond the edges of thechannel region 3 to cover the high resistivity regions 2 and 4, however,it does not cover the source and drain regions. Accordingly, the chargetrapping layer 11 is separated from the high resistivity regions 2 bythe gate insulating layer 6 but it directly contacts the source anddrain regions 1 and 5.

In FIG. 2C, the gate insulating layer covers only the channel region sothat the charge trapping layer 11 is in direct contact with both of thesource and drain regions and the high resistivity regions. Furthermore,the configuration shown in FIG. 2D is a modification of theconfiguration of FIG. 2B (or FIG. 2A) where the portion of the gateinsulating layer which extends beyond the gate electrode is thinned.

In the case of the structure shown in FIG. 2A or 2B, when negativecharges are trapped due to a hot carrier injection in the portion of thegate insulating film near the high resistivity region 2 next to thedrain region 5, (marked with “a” in the figure), the positive chargestrapped in the charge trapping layer 11 neutralize the negative charges.In order that the influence of the positive charges in the trappinglayer 11 extends to the high resistivity region, the gate insulatingfilm should not be so thick. For example, it is preferable that the gateinsulating film be 500 Å or less.

In the case of FIG. 2C, the gate insulating layer does not exist on thehigh resistivity regions 2 and 4. Therefore, the conductivity type ofthe high resistivity region 2 is always a weak N-type because of theexistence of the positive charges trapped in the charge trapping layer.It is desirable that the silicon nitride film is formed by a photo CVDor thermal CVD rather than a plasma CVD in order to avoid a damage tothe active layer by a plasma.

Also, in the case of FIG. 2D, the portion of the gate insulating film 6located on the high resistivity region is etched to become thinner thanthe portion of the gate insulating film located on the channel region inorder to enhance the influence of the positive charges trapped in thecharge trapping layer. This structure is advantageous because thethickness of the gate insulating film on the channel region can be madethick so that there is no danger that the reliability of the gateinsulating film be hindered. Alternatively, the insulating film maycover the entire surface of the source and drain regions.

In the present invention, the high resistivity regions are formed in aself-aligning manner using a gate electrode and an anodic oxide filmformed by anodizing the gate electrode. The thickness of the anodicoxide film can be accurately controlled with a high uniformity. Forexample, it can be made thinner than 1000 Å, alternatively, it can bemade thicker than 5000 Å (for example, 1 μm) if desired. Accordingly,the thickness (width) of the high resistivity region can be freely andaccurately controlled by the use of the anodic oxide film.

Also, there are two types of anodic oxides used in the presentinvention. One is a barrier type anodic oxide while the other is aporous anodic oxide. When a barrier type anodic oxide is to be etched,it is necessary to use an etchant containing a hydrofluoric acid.However, a porous anodic oxide may be etched with a phosphoric acidcontaining etchant. Accordingly, it is possible to etch a porous anodicoxide without damaging silicon or silicon oxide which constitutes theTFT. Also, both of the porous anodic oxide and a barrier type anodicoxide are difficult to be etched by dry etching. In particular, aselection ratio is especially high with respect to silicon oxide.Accordingly, the configuration of FIG. 2B can be obtained in thefollowing manner.

A porous anodic oxide is initially formed at 1 μm, for example, on atleast side surfaces of the gate electrode. A portion of the gateinsulating film which extends beyond the gate electrode is removed byetching using the anodic oxide as a mask, following which the porousanodic oxide is removed. As a result, the gate insulating film extendsbeyond the side edges of the gate electrode by about 1 μm as shown inFIG. 2B. In order to obtain the high resistivity regions 2 and sourceand drain regions 1 and 5, an N-type impurity ion is introduced from anupper portion of the gate electrode. In the case of an impurity ionhaving a certain energy, for example, phosphorous ions of 30 keV, theimpurity distributes in a depth direction in accordance with a Gaussiandistribution and the maximum concentration is at about 100 Å deep froman upper surface. Accordingly, while a region of the active layer onwhich the gate insulating layer is not placed can be added with theimpurity at a relatively higher concentration, the region of thesemiconductor under the extension of the gate insulating film is notadded with the impurity so much because most of the impurity is blockedby the insulating film.

Accordingly, the source and drain regions 1 and 5 doped with theimpurity at a high concentration are formed while the high resistivityregions 2 added with the impurity at a lower concentration by one or twodigits are formed under the extended portion of the gate insulatingfilm. When decreasing the acceleration energy of the impurity ion, theamount of the impurity to be introduced into the high resistivityregions through the insulating film becomes smaller and the conductivitytype thereof becomes the same as that of the channel region.

EXAMPLE 1

manufacturing process of a TFT in accordance with the present inventionwill be explained with reference to FIGS. 3A to 3H.

Initially, a non-alkaline glass substrate 101, for example, Corning 7059(300 mm×400 mm or 100 mm×100 mm) is coated with a silicon oxide film 102of 1000-3000 Å thick. The Silicon oxide film may be formed by sputteringin an oxygen atmosphere. However, it is preferable to use a plasma CVDwith a TEOS (tetra ethoxy silane) gas used as a starting material toimprove the productivity. In place of silicon oxide, other materials maybe used. For example, a single layer of aluminum nitride, a double layerof silicon nitride and aluminum nitride. Aluminum nitride may be formedby a reactive sputtering in a nitrogen atmosphere.

Then, an active layer 103, for example, crystalline silicon is formed onthe silicon oxide film 102. The “crystalline silicon” in this inventionmay be any form of silicon if it includes crystals at least partly, forexample, single crystal, polycrystal or semiamorphous. In this example,an amorphous silicon film is formed to a thickness of 300-5000 Å,preferably, 500-1000 Å through a plasma CVD or LPCVD, and then the filmis crystallized by exposing it in a reducing atmosphere at 550-600° C.for 24 hours. This step can be done using a laser annealing. Finally,the active layer 103 is obtained by patterning the crystallized film.

Then, an insulating film 104 made of silicon oxide for example is formedcovering the active layer 103 to form a gate insulating film. Thethickness of the insulating film is 300-1500 Å, for example, 500 Å orless. A sputtering method can be used to form this film.

On the insulating film 104, an anodizable material is formed for forminga gate electrode. Examples of anodizable materials are aluminum,tantalum, titanium and silicon. These materials may be used in a singlelayer form, respectively. Alternatively, it is possible to use amulti-layer form using two or three of these materials, for example, adouble layered structure consisting of aluminum on which titaniumsilicide is superimposed, or a double layered structure consisting oftitanium nitride on which aluminum is superimposed. The thickness ofeach layer may be varied depending upon the desired device character. Inthis example, an aluminum film containing Si at 1 weight % or Sc at0.1-0.3 weight % formed by electron beam evaporation or sputtering isused.

Further, a film which will function as a mask in a subsequentanodization step is formed on the aluminum film. A photoresist material(e.g. OFPR 800/30 cp manufactured by Tokyo Oka) formed by spin coatingis used as the mask film. Also, it is desirable to form a barrier typeanodic oxide film on the surface of the aluminum film at 100-1000 Åprior to the formation of the photoresist material in order to improvethe adhesivity of the photoresist material. Further, the barrier typeanodic oxide film prevents the formation of a porous anodic oxide on theupper portion of the aluminum in the later step.

Then, the aluminum film together with the photoresist film is patternedinto a gate electrode 105 and a mask film 106 as shown in FIG. 3A.

Referring to FIG. 3B, the gate electrode 105 is supplied with anelectric current in an electrolyte to form a porous type anodic oxidefilm 107 only on the side surface of the gate electrode. The porous typeanodic oxide film can be obtained by using an acid electrolyte, forexample, aqueous solution of 3-20% citric acid, oxalic acid, phosphoricacid, chromic acid or sulfuric acid. The applied voltage is relativelylow, for example, in the range of 10-30 V while the current ismaintained constant. With this low voltage, the porous anodic oxide canbe grown to 0.3-25 μm thick, for example, as thick as 1.0 μm.

In this example, the thickness of the anodic oxide 107 is preferably,from 0.3 to 2 μm, for example, 0.5 μm. The temperature of the oxalicacid is kept 30° C. The applied voltage is 10 V. The anodization isperformed for 20-40 minutes. The thickness of the film is controlled bythe anodization time.

Also, after the formation of the porous anodic oxide 107, it ispreferable to form a barrier type anodic oxide 108 on the side surfaceand the top surface of the gate electrode by performing the anodizationin the following manner.

Namely, after removing the mask 106, the gate electrode is applied withan electric current in an electrolyte. An ethylene glycol solutioncontaining 3-10% tartaric acid, boric acid or nitric acid is used inthis anodization. The temperature of the solution is preferably keptlower than the room temperature (20° C.), for example, about 10° C. toimprove the quality of the film. The thickness of the anodic oxide 108increases in proportion to the increase in the magnitude of the appliedvoltage. When the applied voltage is 150 V, the film is as thick as 2000Å. The thickness of the film should be determined depending upon thedesired size of an offset or overlap region. However, it is necessary toapply more than 250 V voltage to increase the thickness of the filmexceeding 3000 Å. Accordingly, it is preferable that the thickness beless than 3000 Å so that a necessity of using such a high voltage whichis harmful for the TFT characteristics should be avoided. (FIG. 3C)

It should be noted that the barrier type anodic oxide 108 is formedinside the porous anodic oxide 107 even though it is formed after theformation of the porous anodic oxide. The etching rate of the porousanodic oxide by the phosphoric acid etchant is more than ten timeslarger than that of the barrier type anodic oxide. Accordingly, thebarrier type anodic oxide is hardly etched by the phosphoric acidetchant and therefore it protects the aluminum gate electrode whenetching the porous type anodic oxide 107 in the later step.

After the formation of the porous type anodic oxide, the insulating film104 is patterned into the gate insulating film 104′ as shown in FIG. 3Dusing the porous anodic oxide as a mask. The etching depth can bedetermined arbitrarily. That is, the insulating film may be completelyremoved to expose a surface of the active layer as shown in the drawing,or only an upper portion of the insulating film may be removed so thatthe active layer is not exposed. However, it is desirable to completelyetch the insulating film from the view point of productivity, productionyield, uniformity. The portion of the insulating film 104 located belowthe gate electrode and the porous anodic oxide remains the samethickness as it is formed.

The etching of the insulating film 104 is a dry etching using a plasmaand it may be either an isotropic etching or an anisotropic etching(RIE). It is necessary that a selection ratio between silicon andsilicon oxide should be sufficiently large so that the active layershould not be etched so much. A CF₄ gas is used as an etchant in thisexample.

Also, when the gate electrode is mainly formed of aluminum, tantalum ortitanium and the insulating film 104 is mainly formed of silicon oxide,a fluorine containing etchant gas is suitable since the silicon oxide iseasily etched while etching rates of alumina, tantalum oxide andtitanium oxide are sufficiently small. Alternatively, a wet etching mayalso be used using an etchant containing a hydrofluoric acid such as a1/100 hydrofluoric acid.

After the formation of the gate insulating film 104′, the porous anodicoxide 107 is removed by using a phosphoric acid containing etchant. Forexample, a mixed acid including phosphoric acid, acetic acid and nitricacid. An etching rate is about 600 Å/minute. The underlying gateinsulating layer 104′remains without being etched as shown in FIG. 3E.

Thus, the gate insulating layer 104′ is formed, which extends beyond theouter edges of the barrier type anodic oxide 108 by a distance “y” asindicated in FIG. 3D. The distance “y” is determined by the thickness(width) of the porous anodic oxide film 107 in a self-aligning manner asis understood from the foregoing explanation.

Then, referring to FIG. 3F, N-type impurity ions, for example,phosphorous ions are introduced into a portion of the active layer byion doping using the gate electrode 105 with the anodic oxide 108 formedthereon and the extended portion of the gate insulating layer 104′ usedas a mask. The dose is 1×10¹⁴ to 5×10¹⁵ atoms/cm², for example, 2×10¹⁵atoms/cm². The acceleration energy is 10-60 keV, for example, 40 keV.The doping gas is phosphine (PH₃). At this condition, the regions 110and 113 are added with the impurity at a sufficiently high concentrationto form source and drain regions, for example, 1×10²⁰ to 2×10²¹atoms/cm³ while the regions 111 and 112 are added with the impurity at alittle concentration, for example, 1×10¹⁷ to 2×10¹⁸ atoms/cm³ because ofthe existence of the gate insulating film thereon. The impurityconcentration is measured by a secondary ion mass spectrography (SIMS).Also, these concentrations correspond to dose amounts 5×10¹⁴-5×10¹⁵atoms/cm² and 2×10¹³-5×10¹⁴ atoms/cm², respectively. Generally, theconcentration of the impurity in the source and drain regions 110 and113 should be higher than that in the high resistivity regions 111 and112 by 0.5 to 3 digits.

As a result, source and drain regions having a relatively lowerresistivity 110 and 113, and high resistivity regions 111 and 112 areformed.

Subsequently, as shown in FIG. 3G, a silicon nitride film 114 is formedthrough a plasma CVD to a thickness of 200-2000 Å on the entire surface.A mixture of silane (SiH₄) and ammonium (NH₃) at a ratio 1:5 is used.The substrate temperature is 250-400° C., for example, 350° C. If theamount of silane is increased, the silicon nitride contains an excesssilicon, resulting in the formation of trap centers of positive chargesat a higher concentration. However, the insulating property is hinderedif the amount of silane is too increased.

Alternatively, the silicon nitride film may be formed through a lowpressure CVD or by injecting nitrogen ions into the silicon film.

After the formation of the silicon nitride film 114, a XeF excimer(wavelength: 355 nm, pulse width: 40 n sec.) is irradiated in order toactivate the impurity ions introduced into the active layer. Awavelength of the laser should be so selected that the laser light maytransmit through the silicon nitride film.

In place of the excimer laser, other lasers may also be used. However,pulsed lasers are desirable rather than continuous wave lasers (CWlasers) because irradiation time of CW lasers is long and there is adanger that the irradiated film is thermally expanded and peeled off.

As to examples of pulsed laser, there are a laser of an IR light such asNd:YAG laser (Q switch pulse oscillation is preferred), a secondharmonic wave of the Nd:YAG (visible light), and a laser of a UV lightsuch as excimer laser of KrF, XeCl and ArF. When the laser beam isemitted from the upper side of a metal film, it is necessary to selectwavelengths of the laser in order not to be reflected by the metal film.However, there is no problem when the metal film is enough thin. Also,it is possible to emit the laser from the substrate side. In this case,it is necessary to select a laser which can transmit through thesilicon.

Also, instead of the laser annealing, a lump annealing of visible lightor near infrared light may be employed. In such a case, the annealing isperformed in order to heat the surface region to 600-1000° C., forexample, for several minutes at 600° C. or several tens seconds at 1000°C. An annealing with a near infrared ray (e.g. 1.2 μm ) does not heatthe glass substrate so much because the near infrared ray is selectivelyabsorbed by silicon semiconductors. Further, by shortening theirradiation time, it is possible to prevent the glass from being heated.

After the activation of the impurity, hydrogen ions are introduced byion doping into the active layer. The acceleration energy is 10-50 kV,for example, 20 kV. The dose is 1×10¹⁴ to 5×10¹⁵ atoms/cm², for example,1×10¹⁵ atoms/cm², This is carried out because the silicon nitride filmmay not transmit hydrogen therethrough by normal heat annealing.Accordingly, hydrogen can be auto-doped into a region between thechannel region and the source/drain regions. Also, it is desirable tocarry out the ion doping of hydrogen after the laser activation of theadded impurity.

Referring to FIG. 3H, an interlayer insulator 115 is formed bydepositing silicon oxide through a plasma CVD to a thickness of 2000 Åto 1 μm, for example, 3000 Å. Subsequently, contact holes are formedthrough the interlayer insulator and aluminum electrode or wiring 116and 117 are formed therethrough. Then, the entire structure is annealedat 200-400° C. in a nitrogen atmosphere in order to activate thehydrogen atoms introduced in the former step. Thus, the TFT iscompleted.

EXAMPLE 2

This example employs the same process as described in the first exampleuntil the structure shown in FIG. 3E is obtained. Thus, redundantexplanations will be omitted. However, the thickness of the insulatingfilm 104 in this example is thicker than that in the first example. Forexample, the insulating film is 1000-1500 Å thick, for example, 1200 Å,so that a gate leak current can be minimized and it can be endure a highvoltage during the anodic oxidation.

Referring to FIG. 4A which corresponds to FIG. 3E, ion doping ofnitrogen ions is carried out using the gate electrode and the insulatingfilm 104′ as a mask at a dose of 1×10¹⁴-3×10¹⁶ atoms/cm², for example,2×10¹⁵ and with an acceleration voltage of 50-100 kV, for example, 80kV. The acceleration voltage is made so high that the nitrogen ionsalmost pass through the regions 110 and 113 of the active layer on whichthe insulating film 104′ does not exist. Accordingly, the regions 110and 113 are not effectively doped with nitrogen. The concentration ofthe nitrogen is less than 1×10¹⁹ atoms/cm³ when measured by SIMS. On theother hand, in the regions 121 and 122 under the extended portion of thegate insulating film 104′, the concentration of the nitrogen takes itsmaximum, namely 5×10¹⁹-2×10²¹ atoms/cm³ (depending on the depth). Thus,the regions 121 and 122 will function as high resistivity regions.

Then, referring to FIG. 4B, the gate insulating film 104′ is furtherpatterned into the gate insulating film 104″ in a self-aligning mannerwith respect to the barrier anodic oxide 108. Then, a silicon nitridefilm 114 is formed in the same manner as in the first example through aplasma CVD to a thickness of 200-2000 Å, for example, 1000 Å. Further,phosphorous ions are introduced into the active layer by ion doping. Thedose is 5×10¹⁴-5×10¹⁵atoms/cm². The acceleration voltage is 50-100 kV,for example, 80 kV. Phosphine is used as a dopant gas. As a result, theregions 110, 113 and 121, 122 are doped with the same amount ofphosphorous. When measured by SIMS, the concentration of the phosphorousis 1×10²⁰ to 2×10²¹ atoms/cm³ which corresponds to a dose of5×10¹⁴-5×10¹⁵ atoms/cm². However, because of the existence of nitrogen,the regions 121 and 122 have a higher resistivity than the regions 110and 113. Also, the silicon nitride film prevent the surface of theactive layer from being damaged during the ion doping of thephosphorous.

The phosphorous ions and nitrogen ions are activated by a subsequentannealing step, for example, with an excimer laser (wavelength 355 nm,pulse width 40 n sec.). Thereafter, hydrogen ions are introduced by iondoping in the same manner as in the first example.

Finally, referring to FIG. 4C, an interlayer insulator 115 of 3000 Åcomprising silicon oxide is formed by CVD. Aluminum electrode or wiring116 and 117 are formed through contact holes formed in the interlayerinsulator. Further, an annealing at 200-400° C. in a nitrogen atmosphereis conducted. Thus, a TFT in accordance with the second example iscompleted.

Referring to FIG. 5A, an example of a monolithic circuit using the TFTin accordance with the present invention will be described. Themonolithic circuit is used for example as a circuit substrate of anactive matrix liquid crystal device where both of pixel TFTs andperipheral circuits formed of TFTs are integrally formed on a samesubstrate. In the figure, TFTs 1-3 are shown. TFTs 1 and 2 are used asdriver TFTs of which barrier type anodic oxide is 200-2000 Å thick, forexample, 1000 Å. The gate electrode and the high resistivity regionsslightly overlap each other because of a diffraction of impurity ionsduring the ion doping. The drain of TFT1 of N-channel type and the drainof TFT2 of p-channel type are connected with each other through a wiring503. Also, the source of TFT1 is grounded while the source of TFT2 isconnected to a power source so that a CMOS inverter is constructed. Itshould be noted that other types of a CMOS circuit may be used as aperipheral circuit.

On the other hand, the TFT 3 is used as a pixel TFT for driving a pixel.The thickness of the anodic oxide is 1000 Å as well as TFT 1 and TFT 2.However, while the width “y” of the high resistivity regions in TFT1 andTFT2 is as thin as 0.2 μm (for example), the width of the highresistivity regions in TFT 3 is made as thick as 0.4-2 μm, for example,0.5 μm in order to reduce a leak current and a parasitic capacitancebetween the gate and the drain. In order to change the width of the highresistivity region, the thickness of the porous anodic oxide iscontrolled as explained above. For this reason, it is desirable that thegate electrode of each TFT be separated from one another so that theanodic oxidation may be performed with respect to each gate electrode ofthe TFTs, independently.

Also, while the TFTs 1 and 3 are N-channel type, the TFT 2 is P-channeltype. Accordingly, the process of the first and second examples is notsuitable for the formation of the TFT 2. For this reason, while the gateinsulating film 104′ is patterned into the film 104″ as shown in FIG.4C, this step should not be done on the TFT 2 so that the siliconnitride film 114 does not contact the high resistivity regions,directly. If the silicon nitride film directly contacts the highresistivity regions, the positive charges trapped in the silicon nitridefilm invert the conductivity type of the high resistivity regions toN-type, resulting in obstructing the source/drain current. For thisreason, the P-channel TFT has a configuration such as shown in thedrawing.

EXAMPLE 3

Referring to FIG. 6A, in the same manner as in the first example, anunderlying film comprising silicon oxide 102, an island form siliconfilm 103 of 800 Å having a crystallinity, a silicon oxide film 104 of1200 Å, a gate electrode 105 made of aluminum of 200 nm-1 μm thick, anda porous anodic oxide film 107 (3000 Å-1 μm , e.g. 5000 Å thick) on theside surface of the gate electrode are formed on a Corning 7059 glasssubstrate 101.

Further, a barrier type anodic oxide film 108 is formed to a thicknessof 1000-2500 Å in the manner as in the first example. (FIG. 6B).

Using the porous anodic oxide 107 as a mask, the silicon oxide film 104is etched into a gate insulating film 104′. Then, using the barrier typeanodic oxide 108 as a mask, the porous anodic oxide 107 is etched off.Subsequently, an impurity element is introduced by ion doping using thegate electrode with the barrier type anodic oxide formed thereon and thegate insulating film used as a mask, thereby, forming low resistivityimpurity regions 110 and 113 and high resistivity impurity regions 111and 112. The dose amount is 1-5×10¹⁴ atoms/cm². The acceleration voltageis 30-90 kV. The impurity element is phosphorous. (FIG. 6C)

Further, a metal film 123, for example, titanium film of 50-500 Å thickis formed on the entire surface by sputtering. In place of titanium,other metals such as nickel, molybdenum, tungsten, platinum and paradiummay be used. As a result, the metal film 123 is formed in direct contactwith the low resistivity regions 110 and 113. (FIG. 6D)

Then, a KrF excimer laser (wavelength 248 nm, pulse width 20 nsec) isemitted onto the films in order to activate the introduced impurity andmake the metal film react with the silicon of the active layer. Thus,metal silicide regions (titanium silicide) 125 and 126 are formed. Theenergy density of the laser is 200-400 mJ/cm², preferably, 250-300mJ/cm². Also, the substrate is heated to 200-500° C. during the laserirradiation in order to avoid a peeling of the metal film.

Then, the metal film remaining on the gate electrode and the gateinsulating film, without reacting with silicon, is removed by etchingwith an etchant containing a mixture of hydro peroxide, ammonium andwater at a ratio 5:2:2. The metal silicide regions 125 and 126 are thusformed.

Thereafter, the gate insulating film 104′ is etched using the gateelectrode portion as a mask to form a new gate insulating film 104″having a thinner portion (200-500 Å thick) as shown in FIG. 6E. Further,a silicon nitride film 114 is formed on the entire surface by plasma CVDto a thickness of 200-2000 Å. Since the high resistivity regions 111 and112 are covered with the thinner portion of the gate insulating film, adamage caused by the plasma CVD can be avoided.

Thereafter, hydrogen ions are introduced into the active layer by iondoping in the same manner as in the first example.

Finally, referring to FIG. 6F, an interlayer insulator 115 of 2000 Å to1 μm, for example, 3000 Å comprising silicon oxide is formed by CVD.Aluminum electrode or wiring 116 and 117 are formed with a thickness of2000 Å-1 μm, e.g. 5000 Å, through contact holes formed in the interlayerinsulator.

In this example, since the aluminum electrodes 116 and 117 contact atitanium silicide, the interface between the source/drain regions andthe electrodes 116 and 117 can be improved and the reliability of thecontact can be improved. Also, it is preferable to form a titaniumnitride layer between the aluminum electrodes and the titanium silicideas a barrier metal. The sheet resistance of the metal silicide regionsis 10-50 Ω/square.

The sheet resistance of the high resistivity regions 111 and 112 is10-500 kΩ/square.

Also, the silicon nitride film 114 formed on the high resistivityregions 111 and 112 through the gate insulating film prevents mobil ionssuch as sodium from entering from the outside.

Further, the regions 110 and 113 in which the phosphorous impurity isadded at a high concentration approximately coincide with the metalsilicide regions 125 and 126. Furthermore, each boundary between the lowresistivity regions 110 and 113 and the high resistivity regions 111 and112 is coextensive with the side edges of the gate insulating film 104′.Moreover, the side edges of the gate insulating film 104′ are alsocoextensive with the inner side edges of the metal silicide regions 125and 126.

FIG. 5B shows an example of a monolithic circuit using the TFT inaccordance with the process shown in FIGS. 6A through 6F. The monolithiccircuit of FIG. 5B is used for example as a circuit substrate of anactive matrix liquid crystal device where both of pixel TFTs andperipheral circuits formed of TFTs are integrally formed on a samesubstrate. In the figure, TFTs 1-3 are shown. TFTs 1 and 2 are used asdriver TFTs of which barrier type anodic oxide is 200-2000 Å thick, forexample, 1000 Å. On the other hand, the TFT 3 is used as a pixeltransistor of which barrier type anodic oxide is 1000 Å thick also. Thesource or drain electrode of TFT 3 is connected to a pixel electrode 505made of ITO. Reference numeral 506 shows an output terminal of aninverter.

The thickness of the barrier type anodic oxide is so selected that theedge of the gate electrode is aligned with the edge of the source/drainregion, considering a diffusion of added impurity. While the width “y”of the high resistivity regions in TFT1 and TFT2 is as thin as 0.2 μm(for example), the width “y” of the high resistivity regions in TFT 3 ismade as thick as 0.4-5 μm, for example, 0.5 μm. In order to change thewidth of the high resistivity region, the thickness of the porous anodicoxide should be controlled as explained above. For this reason, it isdesirable that the gate electrode of each TFT be separated from oneanother so that the anodic oxide may be performed with respect to eachgate electrode of the TFTs, independently. Because the width of the highresistivity regions in TFT 3 is larger, it is possible to reduce aparasitic capacitance occurring between the gate electrode and the drainwhen applying a voltage.

Also, while the TFTs 1 and 3 are N-channel type, the TFT 2 is P-channeltype. Accordingly, the process of the first and second examples is notsuitable for the formation of the TFT 2 as explained in the formerexample.

Also, the formation of the titanium film may be done before the iondoping of the impurity. In this case, it is advantageous that thetitanium film prevents the surface from being charged up during the iondoping. Also, it is possible to carry out an annealing with laser or thelike after the ion doping step and then form the titanium film. Afterthe titanium forming step, the titanium silicide can be formed by lightirradiation or heat annealing.

EXAMPLE 4

Referring to FIG. 7A, in the same manner as in the first example, anunderlying film comprising silicon oxide 102, a crystalline siliconisland 103, a silicon oxide film 104, a gate electrode 105 made ofaluminum of 2000 Å-1 μm thick, and a porous anodic oxide film 107 (6000Å wide) on the side surface of the gate electrode are formed on aCorning 7059 glass substrate 101. Further, a barrier type anodic oxidefilm 108 is formed in the manner described with reference to the firstexample.

Using the porous anodic oxide 107 as a mask, the silicon oxide film 104is etched to form a gate insulating film 104′. Then, using the barriertype anodic oxide 108 as a mask, the porous anodic oxide 107 is etchedoff. Then, a metal film 123 such as titanium is formed on the entiresurface to a thickness of 50-500 Å by sputtering. (FIG. 7B)

Subsequently, an impurity element is introduced by ion doping. The doseamount is 5×10¹⁴ to 5×10¹⁵ atoms/cm². The acceleration voltage is 10-30keV. The impurity element is phosphorous. Because the accelerationvoltage is enough small, while the regions 110 and 113 are doped withsufficient amount of impurity, the regions 111 and 112 are doped withthe impurity at a lower concentration because of the existence of thegate insulating film. Thus, low resistivity regions (source and drainregions) 110 and 113 and high resistivity regions 111 and 112 areformed. Phosphine (PH₃) is used as a doping gas. (FIG. 7C)

Then, a KrF excimer laser (wavelength 248 nm, pulse width 20 nsec) isemitted onto the films in order to activate the introduced impurity inthe regions 111 and 112 and make the titanium film react with thesilicon of the active layer. Thus, titanium silicide regions 125 and 126are formed. The energy density of the laser is 200-400 mJ/cm²,preferably, 250-300 mJ/cm². Also, it is preferable to heat the substrateto 200-500° C. during the laser irradiation in order to avoid a peelingof the metal film. In place of the laser, a lump annealing with avisible ray or near infrared ray may be used. (FIG. 7D)

Then, the metal film remaining on the gate electrode and the gateinsulating film, without reacting with silicon, is removed by etchingwith an etchant containing a mixture of hydro peroxide, ammonium andwater at a ratio 5:2:2. The metal silicide regions 125 and 126 are thusformed.

Thereafter, the gate insulating film 104′ is patterned by dry etchingusing the gate electrode portion as a mask to form a new gate insulatingfilm 104″ as shown in FIG. 7E. Further, a silicon nitride film 114 isformed on the entire surface by plasma CVD to a thickness of 200-2000 Å.

Thereafter, hydrogen ions are introduced into the active layer by iondoping, following which the structure is annealed in a nitrogenatmosphere. Further, an interlayer insulator 115 of for example, 6000 Åcomprising silicon oxide is formed by CVD. Aluminum electrode or wiring116 and 117 are formed through contact holes formed in the interlayerinsulator as shown in FIG. 7F. Thus, a TFT having a high resistivityregion is completed.

FIG. 5C shows an example in which the TFT in accordance with the fourthexample is used in a pixel of an active matrix liquid crystal device. Inthe figure, a region 507 shows a TFT region, a region 508 shows anauxiliary capacitor for assisting a capacitance of a pixel electrode, aregion 509 shows a contact region between first and second wirings. Asilicon nitride film 512 covers an active silicon layer of the TFT, agate electrode, and wirings 510 and 511 formed on the same plane as thegate electrode (which are all provided with an anodic oxide filmthereon). Further, an interlayer insulator 513 is formed on the siliconnitride film.

The TFT is provided with a source electrode 516 and a drain electrode517. The electrode 517 is connected to a pixel electrode 514 of ITO. Theinterlayer insulator 513 covering the wiring 510 is removed at a region515. The pixel electrode 514 and the wiring 510 face with each otherthrough the anodic oxide film and the silicon nitride film 512 to form acapacitor. In this case, a large capacitance can be obtained with asmall area because the gap between the electrodes is small and thedielectric constants of the silicon nitride and the anodic oxide(aluminum oxide) are large.

EXAMPLE 5

Referring to FIG. 8A, in the same manner as in the first example, anunderlying film comprising silicon oxide 102, a crystallinesemiconductor island 103, for example, silicon semiconductor, a siliconoxide film 104, a gate electrode 105 made of aluminum of 2000 Å-1 μmthick are formed on a Corning 7059 glass substrate 101.

Then, a porous anodic oxide film 107 (6000 Å thick) is formed on theupper and side surfaces of the gate electrode using the same conditionsas in the first example. (FIG. 8B)

Further, a barrier type anodic oxide film 108 is formed between the gateelectrode and the porous anodic oxide. (FIG. 8C).

Subsequently, an N-type impurity element is introduced by ion dopingusing the gate electrode with the barrier type anodic oxide formedthereon used as a mask at a dose of 5×10¹⁴ to 5×10¹⁵ atoms/cm³. Theacceleration voltage is 40-100 keV. Phosphine is used as a dopant gas.Accordingly, low resistivity impurity regions 110 and 113 are formed.The channel region extends beyond the side edges of the gate electrodeby the distance “z” to form an offset region which is substantiallyintrinsic. The distance “z” is determined by the width of the porous andbarrier type anodic oxides 107 and 108. Thus, high resistivity regionsare formed. (FIG. 8D)

Then, the porous type anodic oxide 107 is removed by etching to exposethe surface of the barrier type anodic oxide 108.

Then, a KrF excimer laser (wavelength 355 nm, pulse width 40 nsec) isemitted onto the films in order to activate the introduced impurity. Theenergy density of the laser is 200-400 mJ/cm², preferably, 250-300mJ/cm². Also, the substrate may be heated to 200-500° C. during thelaser irradiation in order to avoid a peeling of the metal film. Also,the step may be carried out by lump annealing with visible ray or nearinfrared ray. Further, a silicon nitride film 114 is formed on theentire surface by plasma CVD to a thickness of 200-2000 Å, for example,1000 Å

Thereafter, hydrogen ions are introduced into the active layer by iondoping, following which the structure is annealed in a nitrogenatmosphere to activate the hydrogen. (FIG. 8E)

Finally, referring to FIG. 8F, an interlayer insulator 115 of 6000 Åcomprising silicon oxide is formed by CVD. Contact holes are formed inthe interlayer insulator and electrodes or wirings 116 and 117 made of amultilayer film of titanium nitride and aluminum are formedtherethrough. Thus, a TFT in accordance with the fourth example of theinvention is completed.

EXAMPLE 6

FIGS. 9A-9F show a manufacturing method of a TFT in accordance with thesixth example of the invention. A silicon oxide film 102 of 3000 Å thickis formed by sputtering or plasma CVD on a glass substrate 101. Anamorphous silicon film is formed by plasma CVD or LPCVD on the siliconoxide film 102 to 500 Å thick, following which the silicon film iscrystallized by heating or laser irradiation. Then, the silicon film ispatterned into an active layer 103 for the insulated gate field effecttransistor. Optionally, the amorphous silicon film may be used as theactive layer without being crystallized. (FIG. 9A)

Then, a silicon oxide film 104 is formed by plasma CVD or reducedpressure thermal CVD to 1000 Å thick as an interlayer insulator.Further, an aluminum film containing 0.18% scandium therein is formed byelectron beam evaporation. Then, the surface of the aluminum film isanodic oxidized to form an oxide layer 127 of as thin as 100 Å in anethylene glycol solution containing 5% tartaric acid.

The aluminum film together with the anodic oxide film is patterned intoan island form aluminum film 105 to form a gate electrode. (FIG. 9B)

Then, a porous anodic oxide film 107 is formed to a width of 6000 Å byanodic oxidation using a 10% citric acid solution. (FIG. 9C)

Subsequently, the dense oxide film 127 is removed, following which ananodic oxidation is again performed using an ethylene glycol solutioncontaining a tartaric acid at 5% in order to form a barrier type anodicoxide film 108.

Using the gate electrode 105, the barrier type anodic oxide 108, and theporous anodic oxide 107 as a mask, a portion of the silicon oxide film104 is etched. (FIG. 9D) p Referring next to FIG. 9E, an N-type impurityelement phosphorous is introduced at a dose of 5×10¹⁴ to 5×10¹⁵atoms/cm² in order to form source and drain regions 110 and 113. At thesame time, lightly doped regions 111 and 112 are formed because of theexistence of the silicon oxide film 104 located thereon. Also, offsetgate regions 128 and 129 are formed which have the same conductivitytype as the channel region, i.e. intrinsic. Thereafter, the addedimpurity is activated by heat treatment, laser irradiation or a stronglight irradiation.

The concentration of the phosphorous in the source and drain regions 110and 113 are higher than that in the lightly doped regions 111 and 112 by2-3 digits. For example, the concentration in the source and drainregions is 1×10²⁰ to 2×10²¹ atoms/cm³ while that in the lightly dopedregions is 1×10¹⁷ to 2×10¹⁸ atoms/cm³.

Referring to FIG. 9F, a silicon nitride film 114 is further formed onthe entire surface. As a method for the formation of the silicon nitridefilm, a plasma CVD may be used. However, a photo CVD or thermal CVD maybe used to improve the surface condition. Also, as a starting material,silane and ammonium, silane and N₂O, or a combination thereof may beused. Dichlorosilane may be used instead of silane. The thickness of thesilicon nitride film 114 is 500-2000 Å, for example, 1000 Å.

After the formation of the silicon nitride film, the laminated structureis treated with a heat annealing of 350° C. for 2 hours in order to curedamages caused to the silicon oxide gate insulating film 104, source anddrain regions 110 and 113 by the impurity adding step. During the heatannealing, the hydrogen contained in the silicon nitride film diffusesso that defects existing on the surface of the silicon oxide film 104and the source and drain regions 110 and 113 can be annealed.

Then, an interlayer insulating film 115 is formed by depositing siliconoxide to 5000 Å thick or larger, through a plasma CVD using TEOS as astarting gas. It should be noted that the silicon oxide film formed fromTEOS tends to trap electrons therein. However, the silicon nitride film114 traps positive charges therein and thus neutralize the electrons.Accordingly, the formation of the silicon nitride film adjacent to asilicon oxide film formed from TEOS is especially advantageous.

EXAMPLE 7

The seventh example of this invention is directed to a liquid crystaldevice using TFTs manufactured in accordance with the present inventionto form a circuit substrate. FIG. 10 shows a diagram of a liquid crystaldevice having a pair of substrates between which a liquid crystal isinterposed, one of which is provided with a semiconductor chip which isusually mounted on a main board of a computer. Thereby, the unit can becompact, light weight and thin.

In the drawing, the reference numeral 15 shows a substrate of the liquidcrystal cell. On the substrate 15, an active matrix circuit 14 is formedwhich comprises a number of pixels, each of which comprises a TFT 11,pixel electrode 12 and an auxiliary capacitor 13. Also, anX-decoder/driver, Y-decoder/driver and XY-divider are formed of TFTs onthe substrate to drive the pixels. Of course, it is possible to use theTFTs described in the previous examples. Further, semiconductor chipsare formed on the substrate through wiring bonding method or COG(chip-on-glass) method. In the drawing, a correction memory, memory, CPUand input port are constituted by these chips. Other chips may also beformed.

The input port is to read a signal input from outside and to convert itinto a display signal. The correction memory is to correct the inputsignal or the like for each pixel depending upon the specificcharacteristics of the active matrix panel. In particular, thecorrection memory comprises a non-volatile memory storing specificinformation of each pixel of the panel. For example, when there is apoint defect in one pixel of an electro-optical device, the pixelsaround the defect are supplied with corrected signals so that the defectappears less. Also, when the brightness in one pixel is lower thanothers, a stronger signal is sent to the pixel in order to compensatethe brightness. Since the defect information of pixels are different ineach panel, the information stored in the correction memory is differentin each panel.

The CPU and the memory have the same functions as those used in aconventional computer. In particular, the memory comprises a RAM havinga display memory in correspondence with each pixel. These chips are allCMOS types.

Also, a part of the foregoing chips may be constituted by TFTs of thepresent invention. The liquid crystal substrate of the present examplehas a CPU and a memory mounted thereon, which is comparable with asimple electronic device like a personal computer. This is veryadvantageous for compacting the liquid crystal display system andextending the application thereof.

The pixel TFT 11 may be formed in the following manner.

Referring to FIG. 11A, a base silicon oxide film 102 is formed bysputtering on a glass substrate 101. Then, an amorphous silicon film isformed through plasma CVD or low pressure thermal CVD to a thickness of500 Å. The amorphous silicon film is crystallized by heating or laserirradiation, following which the film is patterned into an active layer103.

Referring to FIG. 11B, a silicon oxide film 104 is formed to a thicknessof 1000 Å as a gate insulating film through a plasma CVD or sputtering.Further, a gate electrode 105 is formed by depositing an aluminum filmcontaining scandium at 0.18 weight % to a thickness of 6000 Å andpatterning it. Then, the aluminum gate electrode 105 is subjected to ananodic oxidation in an ethylene glycol solution containing 5% tartaricacid, thereby, forming a dense anodic oxide film 108 to a thickness of2000 Å.

Using the gate electrode and the anodic oxide film 108 as a mask,phosphorous ions, an N-type impurity is introduced by plasma doping intoa portion of the active layer 103 to form source and drain regions 110and 113. Because of the existence of the anodic oxide film 108, there isformed a pair of offset regions 128 and 129 between the channel region 3and each of the source and drain regions 110 and 113. After the dopingof the phosphorous ions, the source and drain regions are activated by aheat treatment or a laser or intense light irradiation.

Referring to FIG. 11C, a silicon nitride film 114 is formed by a plasmaCVD using silane and ammonium to a thickness of 1000 Å. Subsequently,the entire structure is heated at 300-500° C., for example, 450° C. inan inert atmosphere. The heating is continued for 1 hour. By thisheating, hydrogen atoms contained in the silicon nitride film diffuseinto the silicon oxide film 104 and cure the defects caused to thesilicon oxide film due to the phosphorous doping step.

Further, a silicon oxide film 115 is formed to a thickness of 5000 Å bya plasma CVD using TEOS and oxygen as a starting gas. The substratetemperature is 300-550° C. during the plasma CVD. Contact holes areformed through the silicon oxide film 115 and source and drainelectrodes 116 and 117 are formed therethrough as shown in FIG. 11D. Thedrain electrode 117 is connected to a pixel electrode 130 made of ITO.

The high resistivity regions of an N channel TFT in accordance with thepresent invention may be either one of an N− type conductivity region, aC, N or O doped region or an offset gate region. Further, two or more ofthem may be combined. In any event, because of the existence of the filmcapable of trapping positive charges adjacent to the high resistivityregion either in direct contact therewith or through a silicon oxidefilm therebetween, it is possible to avoid an occurrence of a parasiticchannel in the high resistivity regions. In particular, the presentinvention is effective to avoid a decrease in mobility when a drainvoltage is several volts. For this reason, when using the N channel TFTas a pixel transistor of a liquid crystal device, it is possible tocontrol delicate voltages and to reproduce an image having delicate grayscales.

Also, the TFTs of the present invention are applicable for TFTs in athree dimensional IC where the TFTs are formed on a substrate formedwith an integrated circuit. The TFTs of the present invention may alsobe formed on a glass or resinous substrate. In any event, the TFTs ofthe present invention are to be formed on an insulating surface.

It is particularly advantageous when the TFTs of the present inventionare used as TFTs of an electro-optical device such as a monolithic typeactive matrix circuit having a peripheral circuit on a same substrate,because the TFTs of the present invention are low in a leak current(Ioff current), are capable of being driven with a higher voltage andhave a higher reliability, which are required for pixel TFTs of anactive matrix circuit.

In place of silicon oxide as a gate insulating film, it is possible touse other materials such as silicon nitride, silicon oxinitride (SiON).Also, a multi-layer of these materials may be used.

Further, the silicon nitride film used in the present invention may havea multi-layer structure. For example, the film comprises first and thirdsilicon nitride layers in which the ratio Si:N is approximately 3:4, anda second silicon nitride layer interposed between the first and thirdlayers. The ratio Si:N in the second film is 10:1 to 10:5. Also, thethickness of the first layer is in the range of 10-100 Å, for example,50 Å, the second layer is 20-200 Å, e.g. 100 Å and the third layer is100-5000 Å, e.g. 500 Å. This structure can be formed by changing theflow rate of the nitrogen containing gas with respect to the siliconcontaining gas during the deposition.

While various examples have been disclosed with respect the preferredembodiment of the invention, it is to be understood that the presentinvention should not be limited to those particular examples, but manymodifications may be made by those ordinary skilled person withoutdeparting the scope of the attached claims.

1. A semiconductor device comprising: a pixel portion comprising a firstthin film transistor; and a driver circuit comprising a second thin filmtransistor, wherein each of the first thin film transistor and thesecond thin film transistor comprising: a silicon island over asubstrate having an insulating surface, the silicon island comprising asource region, a drain region, and a channel region located between thesource region and the drain region; a gate electrode provided adjacentto the channel region with a gate insulating layer interposedtherebetween; an impurity region having a lower concentration of animpurity than concentrations of the impurity in the source region andthe drain region provided between the channel region and at least one ofthe source region and the drain region; a layer comprising siliconnitride over the gate electrode and the silicon island, the layer havinga portion in contact with the gate insulating layer; wherein the gateinsulating layer has a first portion over the channel region and asecond portion over the impurity region, at least a portion of thesecond portion is thinner than the first portion, wherein one of thesource region and the drain region of the first thin film transistor iselectrically connected to a pixel electrode, and wherein the pixelelectrode is formed over the layer comprising silicon nitride.
 2. Asemiconductor device according to claim 1, wherein the gate insulatinglayer comprises silicon oxide.
 3. A semiconductor device according toclaim 1, wherein a thickness of the gate insulating layer is 500 Å orless.
 4. A semiconductor device according to claim 1, wherein the pixelelectrode comprises ITO.
 5. A semiconductor device according to claim 1,wherein the gate electrode and the impurity region of the second thinfilm transistor slightly overlap each other.
 6. A semiconductor deviceaccording to claim 1, wherein a width of the impurity region of thefirst thin film transistor is wider than a width of the impurity regionof the second thin film transistor.
 7. A semiconductor device accordingto claim 1, wherein the first thin film transistor is an N-channel typethin film transistor.
 8. A semiconductor device according to claim 1,wherein the first thin film transistor is a P-channel type thin filmtransistor.
 9. A semiconductor device comprising: a pixel portioncomprising a first thin film transistor; and a driver circuit comprisinga second thin film transistor, wherein each of the first thin filmtransistor and the second thin film transistor comprising: a siliconisland over a substrate having an insulating surface, the silicon islandcomprising a source region, a drain region, and a channel region locatedbetween the source region and the drain region; a gate electrodeprovided adjacent to the channel region with a gate insulating layerinterposed therebetween; an impurity region having a lower concentrationof an impurity than concentrations of the impurity in the source regionand the drain region provided between the channel region and at leastone of the source region and the drain region; and a layer comprisingsilicon nitride over the gate electrode and the silicon island, thelayer having portions in contact with the gate insulating layer, thesource region, and drain region respectively, wherein the gateinsulating layer has a first portion over the channel region and asecond portion over the impurity region, at least a portion of thesecond portion is thinner than the first portion, wherein one of thesource region and the drain region of the first thin film transistor iselectrically connected to a pixel electrode, and wherein the pixelelectrode is formed over the layer comprising silicon nitride.
 10. Asemiconductor device according to claim 9, wherein the gate insulatinglayer comprises silicon oxide.
 11. A semiconductor device according toclaim 9, wherein a thickness of the gate insulating layer is 500 Å orless.
 12. A semiconductor device according to claim 9, wherein the pixelelectrode comprises ITO.
 13. A semiconductor device according to claim9, wherein the gate electrode and the impurity region of the second thinfilm transistor slightly overlap each other.
 14. A semiconductor deviceaccording to claim 9, wherein a width of the impurity region of thefirst thin film transistor is wider than a width of the impurity regionof the second thin film transistor.
 15. A semiconductor device accordingto claim 9, wherein the first thin film transistor is an N-channel typethin film transistor.
 16. A semiconductor device according to claim 9,wherein the first thin film transistor is a P-channel type thin filmtransistor.
 17. A semiconductor device comprising: a pixel portioncomprising a first thin film transistor; and a driver circuit comprisinga second thin film transistor, wherein each of the first thin filmtransistor and the second thin film transistor comprising: a siliconisland over a substrate having an insulating surface, the silicon islandcomprising a source region, a drain region, and a channel region locatedbetween the source region and the drain region; a gate electrodeprovided adjacent to the channel region with a gate insulating layerinterposed therebetween; an impurity region having a lower concentrationof an impurity than concentrations of the impurity in the source regionand the drain region provided between the channel region and at leastone of the source region and the drain region; and a layer comprisingsilicon nitride over the gate electrode and the silicon island, thelayer having a portion in contact with the gate insulating layer,wherein the gate insulating layer covers the channel region, the sourceregion, and the drain region, wherein the gate insulating layer has afirst portion over the channel region and a second portion over theimpurity region, at least a portion of the second portion is thinnerthan a the first portion, wherein one of the source region and the drainregion of the first thin film transistor is electrically connected to apixel electrode, and wherein the pixel electrode is formed over thelayer comprising silicon nitride.
 18. A semiconductor device accordingto claim 17, wherein the gate insulating layer comprises silicon oxide.19. A semiconductor device according to claim 17, wherein a thickness ofthe gate insulating layer is 500 Å or less.
 20. A semiconductor deviceaccording to claim 17, wherein the pixel electrode comprises ITO.
 21. Asemiconductor device according to claim 17, wherein the gate electrodeand the impurity region of the second thin film transistor slightlyoverlap each other.
 22. A semiconductor device according to claim 17,wherein a width of the impurity region of the first thin film transistoris wider than a width of the impurity region of the second thin filmtransistor.
 23. A semiconductor device according to claim 17, whereinthe first thin film transistor is an N-channel type thin filmtransistor.
 24. A semiconductor device according to claim 17, whereinthe first thin film transistor is a P-channel type thin film transistor.25. A semiconductor device comprising: a pixel portion comprising afirst thin film transistor; and a driver circuit comprising a secondthin film transistor, wherein each of the first thin film transistor andthe second thin film transistor comprising: a silicon island over asubstrate having an insulating surface, the silicon island comprising asource region, a drain region, and a channel region located between thesource region and the drain region; a gate electrode provided adjacentto the channel region with a gate insulating layer interposedtherebetween; an impurity region having a lower concentration of animpurity than concentrations of the source region and the drain regionprovided between the channel region and at least one of the sourceregion and the drain region; and a layer comprising silicon nitride overthe gate electrode and the silicon island, the layer having a portion incontact with the gate insulating layer, wherein the gate insulatinglayer extends beyond side edges of the gate electrode to cover theimpurity region and an extending portion of the gate insulating layerover a portion of the impurity region is thinner than a portion of thegate insulating layer over the channel region, wherein one of the sourceregion and the drain region of the first thin film transistor iselectrically connected to a pixel electrode, and wherein the pixelelectrode is formed over the layer comprising silicon nitride.
 26. Asemiconductor device according to claim 25, wherein the gate insulatinglayer comprises silicon oxide.
 27. A semiconductor device according toclaim 25, wherein a thickness of the gate insulating layer is 500 Å orless.
 28. A semiconductor device according to claim 25, wherein thepixel electrode comprises ITO.
 29. A semiconductor device according toclaim 25, wherein the gate electrode and the impurity region of thesecond thin film transistor slightly overlap each other.
 30. Asemiconductor device according to claim 25, wherein a width of theimpurity region of the first thin film transistor is wider than a widthof the impurity region of the second thin film transistor.
 31. Asemiconductor device according to claim 25, wherein the first thin filmtransistor is an N-channel type thin film transistor.
 32. Asemiconductor device according to claim 25, wherein the first thin filmtransistor is a P-channel type thin film transistor.
 33. A semiconductordevice comprising: a pixel portion comprising a first thin filmtransistor; and a driver circuit comprising a second thin filmtransistor, wherein each of the first thin film transistor and thesecond thin film transistor comprising: a silicon island over asubstrate having an insulating surface, the silicon island comprising asource region, a drain region, and a channel region located between thesource region and the drain region; a gate electrode provided adjacentto the channel region with a gate insulating layer interposedtherebetween; an impurity region having a lower concentration of theimpurity than concentrations of the source region and the drain regionprovided between the channel region and at least one of the sourceregion and the drain region; and a layer comprising silicon nitride overthe gate electrode and the silicon island, the layer having portions incontact with the gate insulating layer, the source region, and drainregion respectively, wherein the gate insulating layer extends beyondside edges of the gate electrode to cover the impurity region and anextending portion of the gate insulating layer over a portion of theimpurity region is thinner than a portion of the gate insulating layerover the channel region, wherein one of the source region and the drainregion of the first thin film transistor is electrically connected to apixel electrode, and wherein the pixel electrode is formed over thelayer comprising silicon nitride.
 34. A semiconductor device accordingto claim 33, wherein the gate insulating layer comprises silicon oxide.35. A semiconductor device according to claim 33, wherein a thickness ofthe gate insulating layer is 500 Å or less.
 36. A semiconductor deviceaccording to claim 33, wherein the pixel electrode comprises ITO.
 37. Asemiconductor device according to claim 33, wherein the gate electrodeand the impurity region of the second thin film transistor slightlyoverlap each other.
 38. A semiconductor device according to claim 33,wherein a width of the impurity region of the first thin film transistoris wider than a width of the impurity region of the second thin filmtransistor.
 39. A semiconductor device according to claim 33, whereinthe first thin film transistor is an N-channel type thin filmtransistor.
 40. A semiconductor device according to claim 33, whereinthe first thin film transistor is a P-channel type thin film transistor.41. A semiconductor device comprising: a pixel portion comprising afirst thin film transistor; and a driver circuit comprising a secondthin film transistor, wherein each of the first thin film transistor andthe second thin film transistor comprising: a silicon island over asubstrate having an insulating surface, the silicon island comprising asource region, a drain region, and a channel region located between thesource region and the drain region; a gate electrode provided adjacentto the channel region with a gate insulating layer interposedtherebetween; an impurity region having a lower concentration of animpurity than concentrations of the impurity in the source region andthe drain region provided between the channel region and at least one ofthe source region and the drain region; and a layer comprising siliconnitride over the gate electrode and the silicon island, the layer havinga portion in contact with the gate insulating layer, wherein the gateinsulating layer extends beyond side edges of the gate electrode tocover the source region and the drain region and an extending portion ofthe gate insulating layer over a portion of the impurity region isthinner than a portion of the gate insulating layer over the channelregion, wherein one of the source region and the drain region of thefirst thin film transistor is electrically connected to a pixelelectrode, and wherein the pixel electrode is formed over the layercomprising silicon nitride.
 42. A semiconductor device according toclaim 41, wherein the gate insulating layer comprises silicon oxide. 43.A semiconductor device according to claim 41, wherein a thickness of thegate insulating layer is 500 Å or less.
 44. A semiconductor deviceaccording to claim 41, wherein the pixel electrode comprises ITO.
 45. Asemiconductor device according to claim 41, wherein the gate electrodeand the impurity region of the second thin film transistor slightlyoverlap each other.
 46. A semiconductor device according to claim 41,wherein a width of the impurity region of the first thin film transistoris wider than a width of the impurity region of the second thin filmtransistor.
 47. A semiconductor device according to claim 41, whereinthe first thin film transistor is an N-channel type thin filmtransistor.
 48. A semiconductor device according to claim 41, whereinthe first thin film transistor is a P-channel type thin film transistor.